Automate adding a Surface on a cut Mesh
Try the Bisect Function

3D View > Mesh > Bisect, requires a Face Selection



This allows you to split the mesh along a plane you can define by dragging, or numerically. If your Operator Panel is not open, hit F6 to show it.
It come with a bunch of useful options, like 


Fill, which fills the cut edges with faces
Clear Inner/Outer which remove one or both sides of the cut
Axis Threshold that gives the cut adge some fuzzines in where the cut-off happens.